In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as those associated with boiler systems for providing steam to a power plant, a hot process gas (or flue gas) is generated. Such a flue gas will often contain, among other things, carbon dioxide (CO2). The negative environmental effects of releasing carbon dioxide to the atmosphere have been widely recognized, and have resulted in the development of processes adapted for removing carbon dioxide from the hot process gas generated in the combustion of the above mentioned fuels. One such system and process previously disclosed is the single-stage Chilled Ammonia based system and process for removal of CO2 from a post-combustion flue gas stream.
Known Chilled Ammonia based systems and processes (CAP) provide a relatively low cost means for capturing and removing CO2 from a gas stream, such as, for example, a post combustion flue gas stream. An example of such a system and process has previously been disclosed in the published international patent application WO 2006/022885 titled Ultra Cleaning of Combustion Gas Including the Removal of CO2. WO 2006/022885 discloses a method for removing carbon dioxide from a flue gas, which method includes capturing carbon dioxide from a flue gas cooled to a temperature below ambient temperature (preferably between 0° C. and 20° C., more preferably between 0° C. and 10° C.) in a CO2 absorber by means of an ammoniated solution or slurry. The CO2 is absorbed by the ammoniated solution in the absorber at a temperature between 0° C. and 20° C., more preferably between 0° C. and 10° C., after which the ammoniated solution is regenerated in a regenerator under elevated pressure and temperature to allow the CO2 to escape the ammoniated solution as gaseous carbon dioxide of high purity. In the process described in WO 2006/022885, the regenerated ammoniated solution may be reused in the CO2 absorption process such that a circulating stream of ammoniated solution is formed.
A problem in the chilled ammonia process as described in WO 2006/022885, is that water entering the circulating stream of ammoniated solution, for example as moisture in the incoming flue gas stream, is accumulated in the ammoniated solution. This accumulated water acts to dilute the ammoniated solution, thereby reducing the ability of the ammoniated solution to “capture” CO2 from a flue gas stream.
Furthermore, another problem in the chilled ammonia process as described in WO 2006/022885 is that trace components, i.e. components other than carbon dioxide, ammonia water and the respective ions/salts, captured in the Chilled Ammonia Process may accumulate.
Previously, the amount of water and trace components in the circulating ammoniated solution has been adjusted by, e.g., removing a portion of the ammoniated solution in a “bleed stream” and by compensating the dilution of the ammoniated solution by adding fresh ammonia. The ammoniated solution in the “bleed stream” must then be disposed of and fresh ammonia must be provided, which leads to increased costs and environmental issues.
Another solution is to include an evaporator, hereafter termed appendix stripper, which evaporates virtually all carbon dioxide, most ammonia and a significant amount of water. The evaporated compounds are reused in the process. The appendix stripper requires a fairly large heat input to evaporate the water, e.g., to double the amount of trace components roughly 50% of the water have to be evaporated, which makes the appendix stripper a major heat consumer. In addition, it requires cooling of the overhead stream. Depending on the operating pressure of the appendix stripper, this heat can be (partially) reused at a lower exergy level or has to be rejected by external cooling. High grade stainless steel or other resistant materials of construction are indicated for the appendix stripper due to the corrosive properties exhibited by the media and especially the trace components at increased concentrations, which is aggravated by the elevated temperature, at which the appendix stripper operates.